Stable iodine was measured in the thyroid gland of the chick embryo from day 9 to day 20 of incubation in order to evaluate quantitatively the functional development of the gland. Total iodine content increased progressively during incubation. From day 9 to day 17 of incubation, this increase resulted from the increases of pellet-bound iodine and of soluble iodine. Afterwards, it essentially paralleled the increase of the soluble thyroglobulin-bound iodine which reflected the increase in both thyroglobulin content and the degree of iodination of the thyroglobulin. The total iodine, thyroglobulin-bound iodine and thyroglobulin (TG) content, increased as power functions of time during incubation, with critical times on days 11 and 15. Their concentrations also increased during the whole incubation period, while the iodide concentration remained roughly constant (25 ng/mg) from day 13 to day 19. Only one iodoprotein, 19.5 S TG, was found, and its heterogeneity of iodination was demonstrated during the whole period of incubation studied (from day 11 to day 20). The degree of dissociation with sodium dodecyl sulfate (SDS) of the TG into 12 S subunits decreased as the degree of iodination of the TG increased. Throughout embryonic development, iodine was bound more and more to TG molecules, which were resistant to dissociation with SDS. While the average iodine content of the TG increased, no appreciable changes were found in iodotyrosine and iodothyronine percentages of TG-bound iodine: monoiodotyrosine, 26%; diiodotyrosine, 43%; thyroxine 12%; 3,5,3'-triiodothyronine, 2.5%. As a consequence, a linear relationship existed for each iodoamino acid between the number of its residues per mole of TG and the iodine content of TG (127I atoms per mole)-- about 30 atoms of iodine was required to form 1 mole of T4. The low efficiency of the TG of the chick embryo as a thyroidal hormone-forming protein was compensated for by its high degree of iodination.
Influence of thiourea on the glycogen content of the liver of the chick embryo In the developing chick embryo, liver glycogen appears on the sixth day of incubation. The glycogen content increases from the sixth day, but decreases on the twelfth day before increasing again. This decrease on day 12 might be related to the onset of thyroid activity, which would be responsible for an increased rate of utilization of the substrates. An antithyroid drug, thiourea, has been injected on day 6·0. The liver glycogen concentration of the thiourea-treated chick embryos was determined from the tenth to the fifteenth days and compared with that of the control embryos. (i) On days 11 and 12 the liver glycogen concentration of the injected embryos is higher than that of the controls. (ii) On day 13 the liver glycogen level of treated embryos decreases; this decrease might be correlated with possible functional activity of the thyroid glands if their biosynthesis is no longer inhibited by the thiourea injection or with thyroid hormones possibly coming from the yolk. (iii) On days 14 and 15 no difference is observed between the liver glycogen content of the thiourea-treated embryos and that of the controls.
Thyroid iodine in the developing chick embryo Total thyroidal 127I of chick embryo has been studied from the 7th to the 20th days of incubation. On the 7th and 8th days the thyroid concentrates 127I but this concentration is weak. The total amount of 127I, Q, increases from the 8th day (0·1 ng) to the end of incubation, where its value is nearly 4·6 μg. Between the 12th and 13th days there is an abrupt rise of the daily increment, ΔQ = Q2 – Q1, and of the rate of accumulation, ΔQ/Q1. Between the 17th and 18th days there is a decrease of the increment and the rate of accumulation.
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